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VPCS ignoring default gateway in a VLAN with multiple possible gateway

Pedro Matias
Level 1
Level 1

These are pictures of my network and a few traceroutes: https://imgur.com/a/N378WfV

Basically, PC4 is in a VLAN with R2, R3, R4, R5, PC1 and a few switches. The default gateway selected for it is R4, and you can see so in "show ip" in the picture. However, when checking trace from PC4 to PC2 it ignores the default gateway, and chooses randomly between R3 and R2.

However, when tracing from PC4 to PC3 it seems to always end through R4.

I believe the behaviour I'm getting is that it chooses R4 always when the route through R4 is as good as the best route, but when it isn't it prefers to use a gateway that has a better route.

How can I make it so that a VPC always sends the packets to the default gateway?

1 Accepted Solution

Accepted Solutions

this is not randomly happened, 
the VPC sned to default GW which is R4 , here the R4 will send ICMP redirect to VPC because it see that R2 and R3 have better path to destination. 
how you can force VPC to use only R4 ?
clear arp in VPC 
and config in R4 interface no ip redirect.

View solution in original post

4 Replies 4

balaji.bandi
Hall of Fame
Hall of Fame

Looks for me (we do not have config visibility) .4  is VIP IP address (i am guesssing)

if you running any IGP the traffic will load-balance between the routers as ECMP.

To go deep - you need to provide R2 and R3 also switch - show run, show ip route

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How to Ask The Cisco Community for Help

this is not randomly happened, 
the VPC sned to default GW which is R4 , here the R4 will send ICMP redirect to VPC because it see that R2 and R3 have better path to destination. 
how you can force VPC to use only R4 ?
clear arp in VPC 
and config in R4 interface no ip redirect.

This is exactly what is happening with ping, I checked with wireshark.

Now I have a followup problem: in the switch router R4, I can't disable ICMP redirects in int f1/0 and int f1/1. I'm able to do it in f0/0 and f0/1, but not in the interfaces attached to the LAN. As such, it is still sending out the ICMP redirects.

For context: all the routers are c3725, and they have in slot 0T96100-FE. The switch routers like R4, have NM-16ESW in slot 1.

I have another related question. I tried the following experiment: cleared the ARP table in PC4, and did trace 193.136.10.66, and 193.136.10.66 is the address of PC2. Surprisingly, even though the trace shows that the packets went through R2, which has the address 193.136.10.2, I can't see the ICMP redirects when I use trace. I can always see them with ping. You can see this in the attachments, noticeably in r4_to_b1_link_trace_pc4_to_pc2 you see no redirects, but in r4_to_b1_link_ping_pc4_to_pc2 you can see the redirects.

I checked with wireshark and when I use trace from pc4 to pc2 I can only capture the packets sent to PC4, not the packets sent from PC4. You can see this in the attachments.

However when I use ping from pc4 to pc2 I can capture all packets with Wireshark. You can see so in the attachments

Thank you for your help.

 

 

Screenshot (318).png

I run lab and capture redirect when Traceroute from router to other, 
use filter in wiresharke as following and you can capture the redirect (note you must first clear arp many times in host).
ip.addr == ip