Posts Tagged ‘mtu’

Mar
30

OSPF and MTU Mismatch

Dear Brian,

What is the difference between using the “system mtu routing 1500” and the “ip ospf mtu-ignore” commands when running OSPF between a router and a switch?

Thanks,

Paul

Hi Paul,

Within the scope of the CCIE Lab Exam, it may be acceptable to issue either of these commands to solve a specific lab task. However, it is key to note that there is a difference between ignoring the MTU for the purpose of OSPF adjacency and matching the MTU within a real production network.

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Nov
05

Fragmented IPv4 traffic may cause you a lot of problems in real life. Not only it increases the load on router CPUs, but also impacts applications performance (e.g. TCP needs to re-send the whole packet on a single fragment loss). In addition to that, traffic fragmentation is used in numerous network attacks, allowing an attacker to bypass firewalls or IDSes in some situations. Due to all these reasons, you may want to avoid fragmentation at all and/or ensure your network is insulated from fragmented packets. Unfortunately, there are cases when using IPv4 fragmentation is unavoidable.

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Jan
20

Below are a couple example configurations for PPPoE. Note that you can run into MTU issues when trying to use OSPF over PPPoE. This can easily be resolved by using the “ip ospf mtu-ignore” command as the dialer interface’s MTU is 1492 while the virtual-template’s (virtual-access) MTU is 1500.

*** Client ***
interface Ethernet0/0
 pppoe enable
 pppoe-client dial-pool-number 1
!
interface Dialer1
 ip address 142.1.35.5 255.255.255.0
 encapsulation ppp
 dialer-pool 1
 dialer persistent

*** Server ***

vpdn enable
!
vpdn-group CISCO
 accept-dialin
 protocol pppoe
 virtual-template 1
!
interface Ethernet0/0
 pppoe enable
!
interface Virtual-Template1
 ip address 142.1.35.3 255.255.255.0

The next example is using DHCP to assign the client their IP address:

*** Client ***

interface Ethernet0/1
 pppoe enable
 pppoe-client dial-pool-number 1
!
interface Dialer1
 ip address dhcp
 encapsulation ppp
 dialer pool 1
 dialer persistent

*** Server ***

ip dhcp excluded-address 191.1.45.1 190.12.45.3
!
ip dhcp pool MYPOOL
 network 191.1.45.0 255.255.255.0
!
vpdn enable
!
vpdn-group CISCO
 accept-dialin
 protocol pppoe
 virtual-template 1
!
interface Ethernet0/0
 pppoe enable
!
interface Virtual-Template1
 ip address 191.1.45.5 255.255.255.0
 peer default ip address dhcp-pool MYPOOL

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