01-29-2003 01:28 PM - edited 03-02-2019 04:39 AM
When using a route-map, set next-hop. What happens if the next hop goes down or the link to the next hop goes down? Will the packet be dropped out of the policy and be routed normally, or will it still be sent to a non existent next-hop?
01-29-2003 02:11 PM
It should be routed normally.
You do have the option of specifying two next hops (actually u can specify more that 2...i m not sure abt upper limit). In case the first next-hop specified fails...the packet is routed to second next-hop and so on.
01-29-2003 02:20 PM
If the interface associated with the first next hop specified with the "set ip next-hop" command is down, the optionally specified second or third IP addresses are tried in turn. If the link is down but there are no other set statements, packets are routed normally (i.e. the router uses policy routing first and then the routing table).
For eg.
route-map traffic_source permit 10
match ip address 101
set ip next-hop 172.16.x.x 172.32.x.x
If the next hop goes down but the link is still up, the router will continually ARP for that downed router (i.e. you are black holed).
An alternative is the command "set ip next-hop verify-availability" which will verify the next hop is up using CDP. See link: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fswtch_r/xrfscmd4.htm#1035164
From this link in regards to if the host is down but the link up: "If this command is not set, the packets are either successfully policy routed or remain forever unrouted."
Configuring the "set ip default next-hop" command causes the system to use the routing table first and if the route isn't there then policy route to the specified next hop.
The "set" order of operation is: set ip next-hop, set interface, set ip default next-hop, and then set default interface.
Hope it helps.
Steve
04-30-2003 01:20 AM
Hi Steve,
i wanted to ask something relevant, but i think i'm loosing something in your words...
So, in your exmple if 172.16.x.x is down, but the link is up, will172.32.x.x be used instead?
What about if 172.16.x.x is a PC running WIN2K and ICS (aka NAT) and the internet connection (not the 172.16.x.x) of the PC fails?
What happens then?
Are the packets sent back from the PC to the router and then rerouted to 172.32.x.x?
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