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Author Topic: ERS 5510 DHCP-Relay issues  (Read 1880 times)

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Offline Alessan

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ERS 5510 DHCP-Relay issues
« on: February 20, 2016, 03:42:45 AM »
Hi,

I'm testing dhcp relay on an ERS 5510-24t 6.3.5:

Only two host connected to the switch:
  DHCP Server W2012 on port 24 (vlan 1)
  W7 on port 1 (vlan 26)

  W7 on any other (vlan1) port get ip as usually.
  W7 on port 1 (vlan 26) dont get dhcp, the server receive the discover packet but dont send offer (wireshark).
  W7 on port 1 (vlan 26) with static ip can ping dhcp server.

  if i set a static route on dhcp server: 192.168.2.0 MASK 255.255.255.0 10.8.3.23 all works.... but i think this should not be need it.

  Can anyone help me? Thanks

DHCP Server W2012:
  ip/mask: 10.8.8.71 255.248.0.0
  gw: 10.15.255.254

  Scope1: 10.13.0.0 - 10.13.255.255 gw: 10.15.255.254
  Scope2: 192.168.2.10 - 192.168.2.100 gw: 192.168.2.1
 
ERS 5510:
  ip default-gateway 10.15.255.254
  ip address switch 10.8.3.23
  ip address netmask 255.248.0.0

  vlan create 26 type port 1
  vlan name 26 "TEST"
  vlan members 1 2-24
  vlan members 26 1
  vlan ports 1 pvid 26

  ip routing
  ip route 0.0.0.0 0.0.0.0 10.15.255.254

  interface vlan 26
  ip address 192.168.2.1 255.255.255.0 2
  ip dhcp-relay
  exit
 
  ip dhcp-relay fwd-path 192.168.2.1 10.8.8.71


Offline Johan Witters

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    • BKM Networks
Re: ERS 5510 DHCP-Relay issues
« Reply #1 on: February 22, 2016, 06:07:38 AM »
Hi Allessan,

The DHCP server needs a route to subnet for vlan 26. So if the default gateway for the server isn't the switch then yes, you'll need to configure a static route pointing to the switch.
Kind regards,

Johan Witters

Network Engineer
BKM NV

Offline pat2012

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Re: ERS 5510 DHCP-Relay issues
« Reply #2 on: February 22, 2016, 08:05:04 AM »
Just enable a routing protocol on the 5510 (e.g. RIP) and the server would automatically learn the route to the VLAN 26.  Otherwise a static route would be needed as Johan said.

Offline Alessan

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Re: ERS 5510 DHCP-Relay issues
« Reply #3 on: February 22, 2016, 11:19:42 AM »
The unicast relayed packet cames from 10.8.3.23 to 10.8.8.71, but the response go to the relayed agent ip 192.168.2.x... then have sense a route is needed.

This morning i connect this switch to our network:
  both switch and dhcp server have same gateway: 10.15.255.254.
  10.15.255.254 is a router with a static route to 192.168.2.0/24 --> 10.8.3.23 (test-switch)
 
  - dhcp server (10.8.8.71) can ping vlan 26 ip address of the switch (192.168.2.1)
  - a pc with static ip on vlan 26 (192.168.2.x) can ping dhcp server (10.8.8.71)
  - dhcp server dont try to send response to discover packets from vlan 26  :(

  discover...
  discover...
  discover...

  (again with static route works)

PD: I try activating rip on 5510 but server dont learn any route

Any sugestion? I really hate to have to create a .bat to add static routes on startup

Edit: maybe superscope is needed?
« Last Edit: February 22, 2016, 12:46:31 PM by Alessan »

Offline Johan Witters

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    • BKM Networks
Re: ERS 5510 DHCP-Relay issues
« Reply #4 on: February 23, 2016, 11:18:58 AM »
I think your problem is related to asymmetric routing:

request: client -> switch -> DHCP server
Offer:     DHCP server -> router -> switch -> client

because the discover is sent from the switch, the offer should be sent back to the switch. It is not, due to the gateway being pointed to the firewall/router. Probably the router drops the packets, or the switch drops it because it is coming from the firewall/router.

Either use your router as gateway for the other vlan, or configure static routes on the DHCP router. I you configure it as persistent (route add -p ) you should not have to configure a batch file.
Kind regards,

Johan Witters

Network Engineer
BKM NV

Offline Alessan

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Re: ERS 5510 DHCP-Relay issues
« Reply #5 on: February 24, 2016, 02:38:53 PM »
Thanks for response Johan,

Wireshark on DHCP server shows that server dont send Offer unless persistent route is added, the offer is not rejected, symply is not sended.

The router are two switches with VLAG and VRRP at core layer, the problem is we reach the limit of 15 VRRP interfaces at core layer and more vlans are required, the only way i see is make local vlans on avaya access switch stacks and let the stack do the high avaliability of gateway (dirty solution, some vlan will have gateway on core layer and some will have gateway on access switch)

Tomorrow i try to remove vlan 1 and add other vlan to force symetric route (swicth and dhcp server not in same vlan).

Thanks to all