This publication briefly covers the use of 3rd party next-hops in OSPF, RIP, EIGRP and BGP routing protocols. Common concepts are introduced and protocol-specific implementations are discussed. Basic understanding of the routing protocol function is required before reading this blog post.
Third-party next-hop concept appears only to distance vector protocol, or in the parts of the link-state protocols that exhibit distance-vector behavior. The idea is that a distance-vector update carries explicit next-hop value, which is used by receiving side, as opposed to the “implicit” next-hop calculated as the sending router’s address – the source address in the IP header carrying the routing update. Such “explicit” next-hop is called “third-party” next-hop IP address, allowing for pointing to a different next-hop, other than advertising router. Intitively, this is only possible if the advertising and receiving router are on a shared segment, but the “shared segment” concept could be generalized and abstracted. Every popular distance-vector protocols support third party next-hop – RIPv2, EIGRP, OSPF and BGP all carry explicit next-hop value. Look at the figure below – it illustrates the situation where two different distance-vector protocols are running on the shared segment, but none of them runs on all routers attached to the segment. The protocols “overlap” at a “pivotal” router and redistribution is used to provide inter-protocol route exchange.
Per the default distance-vector protocol behavior, traffic from one routing domain going into another has cross the “pivotal” router, the router where the two domains overlap (R3 in our case) – as opposed to going directly to the closes next-hop on the shared segment. The reason for this is that there is no direct “native” update exchange between the hops running different routing protocols. In situations like this, it is beneficial to rewrite the next-hop IP address to point toward the “optimum” exit point, using the “pivotal” router’s knowledge of both routing protocols.
OSPF is somewhat special with respect to the 3rd party next-hop implementation. It supports third-party next-hop in Type-5/7 LSAs (External Routing Information LSA and NSSA External LSA). These LSAs are processed in “distance-vector manner” by every receiving router. By default, the LSA is assumed to advertise the external prefix “connected” to the advertising router. However, if the FA is non-zero, the address in this field is used to calculate the forwarding information, as opposed to default forwarding toward the advertising router. Forwarding Address is always present in Type-7 LSAs, for the reason illustrated on the figure below:
Since there could be multiple ABRs in NSSA area, only one is elected to perform 7-to-5 LSA translation – otherwise the routing information will loop back in the area, unless manual filtering implemented in the ABRs (which is prone to errors). Translating ABR is elected based on the highest Router-ID, and may not be on the optimum path toward the advertising ASBR. Therefore, the forwarding address should prompt the more optimum path, based on the inter-area routing information.
We start with the scenario where we redistribute RIP into EIGRP.
Notice that EIGRP will not insert the third-party next-hop until you apply the command no ip next-hop-self eigrp on R3′s connection to the shared segment. Look at the routing table output prior to applying the no ip next-hop-self eigrp command.
R1#show ip route eigrp 184.108.40.206/16 is variably subnetted, 2 subnets, 2 masks D EX 220.127.116.11/32 [170/2560002816] via 18.104.22.168, 00:00:27, FastEthernet0/0
After the command has been applied to R3’s interface:
R1#show ip route eigrp 22.214.171.124/16 is variably subnetted, 2 subnets, 2 masks D EX 126.96.36.199/32 [170/2560002816] via 188.8.131.52, 00:00:04, FastEthernet0/0
The same behavior is observed when redistributing OSPF into EIGRP, but not when redistributing BGP. For some reason, BGP’s next-hop is not copied into EIGRP, e.g. in the example below, EIGRP will NOT insert the BGP’s next-hop into updates. Notice that you may enable or disable the third-party next-hop behavior in EIGRP using the interface-level command ip next-hop-self eigrp.
RIP passes the third-party next-hop from OSPF, BGP or EIGRP. For instance, assume EIGRP redistribution into RIP. You have to turn on the no ip split-horizon on R3′s Ethernet connection to get this to work:
R2#show ip route rip 184.108.40.206/16 is variably subnetted, 3 subnets, 2 masks R 220.127.116.11/32 [120/1] via 18.104.22.168, 00:00:17, FastEthernet0/0
Notice the following RIP debugging output, which lists the third-party next-hop:
RIP: received v2 update from 22.214.171.124 on FastEthernet0/0 126.96.36.199/32 via 188.8.131.52 in 1 hops 184.108.40.206/24 via 0.0.0.0 in 1 hops
Surprisingly, there is NO need to enable the command no ip split-horizon on the interface when redistributing BGP or OSPF routes into RIP. Seem like only EIGRP to RIP redistribution requires that. Keep in mind, however, that split-horizon is OFF by default on physical frame-relay interfaces. Here is a sample output of redistributing BGP into RIP using the third-party next-hop:
R3#show ip route bgp 220.127.116.11/16 is variably subnetted, 3 subnets, 2 masks B 18.104.22.168/32 [20/0] via 22.214.171.124, 00:22:13 R3# R1#show ip route rip 126.96.36.199/16 is variably subnetted, 3 subnets, 2 masks R 188.8.131.52/32 [120/1] via 184.108.40.206, 00:00:09, FastEthernet0/0
RIP’s third-party next-hop behavior is fully automatic. You cannot disable or enable it, like you do in EIGRP.
Similarly to RIP, OSPF has no problems picking up the third-party next-hop from BGP, EIGRP or RIP. Here is how it would look like (guess which protocol is redistributed into OSPF, based solely on the commands output):
R1#sh ip route ospf 220.127.116.11/16 is variably subnetted, 3 subnets, 2 masks O E2 18.104.22.168/32 [110/1] via 22.214.171.124, 00:34:59, FastEthernet0/0 R1#show ip ospf database external OSPF Router with ID (126.96.36.199) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 131 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 188.8.131.52 (External Network Number ) Advertising Router: 184.108.40.206 LS Seq Number: 80000002 Checksum: 0xF749 Length: 36 Network Mask: /32 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 1 Forward Address: 220.127.116.11 External Route Tag: 200
If you’re still guessing, the external protocol is BGP, as could have been seen observing the automatic External Route Tag – OSPF set’s it to the last AS# found in the AS_PATH.
There are special conditions to be met by OSPF for the FA address to be used. First, the interface where the third party next-hop resides should be advertised into OSPF using the network command. Secondly, this interface should not be passive in OSPF and should not have network type point-to-point or point-to-multipoint. Violating any of these conditions will stop OSPF from using the FA in type-5 LSA created for external routes. Violating any of these conditions prevents third-party next-hop installation in the external LSAs.
OSPF is special in one other respect. Distance vector-protocols such as RIP or EIGRP modify the next-hop as soon as they pass the routing information to other devices. That is, the third party next-hop is not maintained through the RIP or EIGRP domain. Contrary to these, OSPF LSAs are flooded within their scope with the FA unmodified. This creates interesting problem: if the FA address is not reachable in the receiving router’s routing table, the external information found in type 7/5 LSA is not used. This situation is discussed in the blog post “OSPF Filtering using FA Address”.
When you redistribute any protocol into BGP, the system correctly sets the third-party next-hop in the local BGP table. Look at the diagram below, where EIGRP prefixes are being redistributed into BGP AS 300:
R3’s BGP process installs R1 Loopback0 prefix into the BGP table with the next-hop value of R1’s address, not “0.0.0.0” like it would be for locally advertised routes. You will observe the same behavior if you inject EIGRP prefixes into BGP using the network command.
R3#sh ip bgp BGP table version is 9, local router ID is 18.104.22.168 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 22.214.171.124/32 126.96.36.199 156160 32768 ?
Furthermore, BGP is supposed to change the next-hop to self when advertising prefixes over eBGP peering sessions. However, when all peers share the same segment, the prefixes re-advertised over the shared segment do not have their next-hop changed. See the diagram below:
Here R1 advertises prefix 188.8.131.52/24 to R3 and R3 re-advertises it back to R2 over the same segment. Unless non-physical interfaces are used to form the BGP sessions (e.g. Loopbacks), the next-hop received from R1 is not changed when passing it down to R2. This implements the default third-party next-hop preservation over eBGP sessions. Look at the sample output for the configuration illustrated above: R1 receives R2’s prefix with unmodified next-hop.
R1#show ip bgp BGP table version is 3, local router ID is 184.108.40.206 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 220.127.116.11/32 0.0.0.0 0 32768 i *> 18.104.22.168/32 22.214.171.124 0 300 200 i
There is a way to disable this default behavior in BGP. A logical assumption would be that using the command neighbor X.X.X.X next-hop-self would work, and it does indeed, in the recent IOS versions. The older IOS, such as 12.2T did not have this command working for eBGP sessions, and your option would have been using a route-map with set ip next-hop command. The route-map method may still be handy, if you want insert totally “bogus” IP next-hop from the shared segment – receiving BGP speaker will accept any IP address that is on the same segment. That is not something you would do in the production environment too often, but definitely an interesting idea for lab practicing. One good use in production is changing the BGP next-hop to an HSRP virtual IP address, to provide physical BGP speaker redundancy. Here is a sample code for setting an explicit next-hop in BGP update:
router bgp 300 neighbor 126.96.36.199 remote-as 100 neighbor 188.8.131.52 route-map BGP_NEXT_HOP out ! route-map BGP_NEXT_HOP permit 10 set ip next-hop 184.108.40.206
All popular distance-vector protocols support third-party next-hop insertion. This mechanism is useful on multi-access segments, in situations when you want pass optimum path information between routers belonging to different routing protocols. We illustrated that RIP implements this function automatically, and does not allow any tuning. On the other hand, EIGRP supports third-party next-hop passing from any protocol, other than BGP, and you may turn this function on/off on per-interface basis. Furthermore, OSPF’s special feature is propagation of the third-party next-hop within an area/autonomous system, unlike the distance-vector protocols that reset the next-hop at every hop (considering AS a being a “single-hop” for BGP). Thanks to that feature, OSPF offers interesting possibility to filter external routing information by blocking FA prefix from the routing tables. Finally, BGP gives most flexibility when it comes to the IP next-hop manipulation, allowing for changing it to any value.
About Petr Lapukhov, 4xCCIE/CCDE:
Petr Lapukhov's career in IT begain in 1988 with a focus on computer programming, and progressed into networking with his first exposure to Novell NetWare in 1991. Initially involved with Kazan State University's campus network support and UNIX system administration, he went through the path of becoming a networking consultant, taking part in many network deployment projects. Petr currently has over 12 years of experience working in the Cisco networking field, and is the only person in the world to have obtained four CCIEs in under two years, passing each on his first attempt. Petr is an exceptional case in that he has been working with all of the technologies covered in his four CCIE tracks (R&S, Security, SP, and Voice) on a daily basis for many years. When not actively teaching classes, developing self-paced products, studying for the CCDE Practical & the CCIE Storage Lab Exam, and completing his PhD in Applied Mathematics.
5 Responses to “Understanding Third-Party Next-Hop”
Leave a Reply