Route Caching

Unanswered Question
Oct 18th, 2008
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Remote site connected to Head Office over MPLS Link (Primary)

Remote site connected to Head Office over DSL/VPN to ASA5520 (Backup)


Remote router: Cisco 2821

Exchange Server: Win 2003



Problem:


When MPLS link on remote router (2821) drops and the DSL/VPN comes up, the customer cannot reach the

Exchange server in Head Office. All other LAN devices (servers/switches/pc) can be reached.


On the Exchange server, 'route print' displays that the server is caching the remote site clients IP address with the Default gateway of the MPLS router(Head Office), using a metric of 1.



The only way to fix this issue at the moment, is to delete the remote client route from the Exchange server routing tables


route delete xxxx



Can you disable Win 2003 from learning remote routes? Any ideas



Colm







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Giuseppe Larosa Sat, 10/18/2008 - 03:36
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Hello Colm,

another solution could be that of moving the exchange servers behind a pair of routers offering an HSRP VIP so that from its point of view nothing changes when the MPLS link is down.


put it in net 192.168.11.0 so that is never aware of topologies changes on remote sites

its gw is always 192.168.11.1 in any condition.


You can use SVI vlans on multilayer switches to do this you don't need to deploy dedicated routers


Hope to help

Giuseppe



colmgrier Mon, 10/20/2008 - 02:21
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Hi Giuseppe,


Attached is the LAN diagram for the customer.


The Exchange & DC servers (Win 2003) are connected to a 3com Stack. The Core switches are 3560 running HSRP.

The issue is the Exchange server is caching routes for remote sites and this is not happening on the DC server.

Both server are connected to the 3com stack and have the same default gateway.


Do Exchange server learning dynamic routes (Route Database). Can this service be disabled.



Regards,

Colm




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Giuseppe Larosa Mon, 10/20/2008 - 03:49
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Hello Colm,

I would use a different Vlan and IP subnet betweeen the MPLS router, ASA and the two V-routers: if you use a different Vlan/IP subnet like 192.168.2.0/24 for the servers nothing changes.


You can also check the ip proxy-arp settings of all the routers and the ASA.

And if ip proxy-arp is enabled check on the servers:


if they have defined the gateway or gatway = their IP address.


With proxy-arp enabled if the server makes an ARP request for the net of a remote site the MPLS router or the ASA could answer.


On the V-routers :


int vlan X


no ip proxy-arp

no ip redirect


on MPLS router and ASA


no ip proxy-arp


if no routing protocol is running on exchange servers they can build the table just using the ARP replies.


Hope to help

Giuseppe



colmgrier Mon, 10/20/2008 - 03:58
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Hi Giuseppe,


Thanks for the feedback and information. It is not possible to change the network design for this customer using different VLANS at the present time.


I will apply the below commands on the all Cisco devices on the LAN and hopefully this will resove the issue.


Thanks,

Colm

colmgrier Sun, 10/26/2008 - 12:32
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Hi Giuseppe,


The above fix resolved the issue. The Exchange server is no longer caching remote site routes.


Thanks again.

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